This invention relates to a texturing method for forming texturing marks on the surface of a substrate for a magnetic hard disk and to slurry for using in such a method.
Magnetic hard disks are being used as a medium for recording data such as sound and image for data recording and reproducing apparatus such as computers. A magnetic hard disk is generally produced by mirror-polishing the surface of a non-magnetic substrate such as a glass substrate or an aluminum substrate with Ni—P plating, carrying out a texturing process on its surface to form approximately concentric circular texturing marks thereon and sequentially forming a magnetic layer, a protective layer, etc. on this textured surface by using a known thin-film technology such as sputtering.
As known to persons skilled in the art, the texturing process is for forming approximately concentric circular line marks on the surface of a magnetic hard disk similar to the texturing marks formed on the surface of the substrate in order to prevent the adsorption of the magnetic head to the magnetic hard disk.
For the purpose of carrying out magnetization of a magnetic hard disk accurately for accurate recording and reproduction, the surface of the magnetic hard disk must satisfy the following four topological conditions.
(1) Firstly, the pitch of the line marks formed on the surface of the magnetic hard disk must be made smaller. In other words, if the number of line marks per unit length in the radial direction of the disk is increased, the number of protruding parts of the linear marks per unit area facing the magnetic head (or the surface portions of the magnetic hard disk near the magnetic head) increases such that it becomes possible to carry out the magnetization of the magnetic hard disk more accurately. In recent years, the number of line marks per unit length in the radial direction of the disk, or the line density, is coming to be required to be 40 lines/μm or more.
(2) Secondly, deep indentations (deep indentations of line marks and scratches) must not be formed on the surface of the magnetic hard disk. This is because, if these indented parts are too deep, magnetic flux from the magnetic head does not reach the magnetic layer near the bottom of the indented parts and cannot magnetize these parts. This makes accurate recording and reproduction impossible. It is also because a magnetic layer may fail to be formed near the bottom of the indented parts at the time of forming a thin film by sputtering.
(3) Thirdly, abnormal protrusions reaching the floating distance of the magnetic head must not be formed such that the magnetic head can fly at a low height. This is because, if the magnetic head collides with such protrusions, the magnetic head may be damaged and the pieces of the protrusions will become attached to the surface of the magnetic hard disk such that accurate recording to and reproduction from the magnetic hard disk become impossible. In recent years, floating distances of 10 nm or less are being required.
(4) Fourthly, the surface roughness of the magnetic hard disk must be made low such that the magnetic head can slide smoothly on the surface of the magnetic hard disk after landing thereon and before floating up therefrom.
In summary, it is required to form line marks having indentations with an appropriate depth and protrusions with an appropriate height, and such topological surface conditions of a magnetic hard disk depends largely on the texturing process carried out on the surface of its substrate.
In view of the above, it is coming to be required in recent years to make the average surface roughness of the substrate within a range between 3 Å and 6 Å and to form texturing marks with line density of 40 lines/μm or greater on the substrate surface by a texturing process, and it is further being required that such texturing marks be formed uniformly and clearly on the substrate surface.
As described in Japanese Patent Publication Tokkai 2005-131711, texturing is conventionally carried out by using free abrading particles, or by supplying slurry having abrading particles dispersed therein to the surface of the substrate and pressing a texturing tape onto the surface of the substrate so as to form approximately concentric circular texturing marks on the substrate surface.
Porous tapes made of a woven or non-woven cloth material or a foamed material capable of taking in polishing debris generated by the texturing process and acting elastically on the substrate surface are commonly used as the texturing tape.
As described in Japanese Patent Publications Tokkai 6-121923 and 2005-131711, furthermore, texturing slurry containing diamond clusters as abrading particles is being used. Diamond clusters mean a bunched combination of particles of artificial diamond obtained by an explosion shock method such as described in Japanese Patent Publications Tokkai 6-121923 and 2005-131711, by Eiji Oosawa in “Details of nano-diamond by oxygenless explosion method,” Journal of Toryu Gakkai, Vol. 47, No. 8 (August, 2003) at pages 414-417 and by Kotaro Hanada in “Characteristics of diamond clusters and their application to solid lubrication,” ibid., Vol. 47, No. 8 (August, 2003) at pages 422-425.
Mechanism of texturing by diamond clusters will be explained first. Diamond clusters dispersed as polishing materials in texturing slurry are carried inside the surface portion of the texturing tape being pressed against the surface of the magnetic hard disk substrate during the texturing process and are thereby pressed against the substrate surface. Larger diamond clusters break up as they are carried in the surface portion of the texturing tape and become diamond clusters of an appropriate size while being pressed against the substrate surface. These diamond clusters act on and texture the substrate surface while being carried inside the surface portion of the texturing tape.
Since diamond clusters are formed with extremely small artificial diamond particles with primary particle diameter of 20 nm or smaller, it used to be considered possible to reduce the average surface roughness of the substrate and to form texturing marks at a high line density by highly purifying the diamond clusters (that is, by making the purity of diamond clusters closer to that of natural diamond).
If texturing slurry having a polishing material comprising diamond clusters is used to texture a substrate for a magnetic hard disk, it is possible to form texturing marks at a line density of 40 lines/μm or more on the substrate surface but spots appear, the texturing marks being not uniform and becoming locally unclear. Thus, the current problem is that there are fluctuations in the quality of the products after the texturing process and that it is not possible to product products of a specified quality level.
In the technical field of texturing, therefore, the development of a new technology is being required for forming texturing marks uniformly and clearly on the surface of a substrate for a magnetic hard disk with average surface roughness in the range of between 3 Å and 6 Å and line density of 40 lines/μm or greater without generating any spots.